Chemical vapor deposition methods and apparatus

ABSTRACT

The present invention provides methods and apparatus for vaporizing and transporting precursor molecules to a process chamber for deposition of thin films on a substrate. The methods and apparatus include CVD solvents that comprise ionic liquids. The ionic liquids comprise salt compounds that have substantially no measurable vapor pressure (i.e., less than about 1 Torr at about room temperature), exhibit a wide liquid temperature range (i.e., greater than about 100° C.), and have low melting points (i.e., less than about 250° C.). A desired precursor is dissolved in a selected CVD solvent comprising an ionic liquid. The solvent and precursor solution is heated to or near the precursor volatilization temperature of the precursor. A stream of carrier gas is directed over or is bubbled through the solvent and precursor solution to distill and transport precursor molecules in the vapor phase to a deposition chamber. Conventional deposition processes may be used to deposit the desired thin film on a substrate.

TECHNICAL FIELD

[0001] The present invention concerns methods for vapor deposition, andparticularly concerns methods for providing volatile precursor moleculesto form a thin film on a substrate via vapor deposition.

BACKGROUND OF THE INVENTION

[0002] Chemical vapor deposition (CVD) is one process for forming thinfilms on semiconductor wafers, such as films of elemental metals orcompounds. CVD involves the formation of a non-volatile solid film on asubstrate by the reaction of vapor phase reactants (precursors) thatcontain desired components of the film. Standard CVD processes use aprecursor source in a vaporization chamber of a CVD apparatus. Thevaporization chamber is connected to a process (or reactor) chamberwherein a deposition substrate, such as a semiconductor wafer, islocated.

[0003] CVD (and other thin film vapor deposition) techniques requiredelivery of a controlled mass of the precursor in the vapor phase.Precise control over the mass of the precursor delivered to the processchamber is needed to form a uniform layer of the desired thin film. Inaddition, the manner of delivery of the precursor must avoiddecomposition of the reactive volatile precursor molecules and must notinclude unwanted volatized elements or compounds.

[0004] Conventional methods of providing a source of vapor-phaseprecursor molecules include (1) direct vaporization of the precursorfrom neat solids or liquids, (2) direct vaporization of a solventcontaining the precursor, and (3) distilling precursor molecules from asolvent by bubbling a carrier gas through a volume of the solventcontaining the precursor.

[0005] Bulk sublimation of a solid precursor and transport of thevaporized solid precursor to the process chamber using a carrier gas hasbeen practiced. However, it is difficult to vaporize a solid at acontrolled rate such that a constant and reproducible flow of vaporizedsolid precursor is delivered to the process chamber. Lack of control ofthe rate of delivery of a vaporized solid precursor is (at least inpart) due to a changing surface area of the bulk solid precursor as itis vaporized. The changing surface area of the solid precursor when itis exposed to sublimation temperatures produces a continuously changingrate of vaporization. This is particularly true for thermally sensitivecompounds. The changing rate of vaporization thus results in acontinuously changing concentration and non-reproducible flow ofvaporized precursor delivered for deposition in the process chamber. Asa result, film growth rate and the composition of films deposited usingsuch techniques are not adequately controlled. Further, sublimation ofsolid precursors requires exposure of the precursor to temperaturesgreater than the vaporization temperature. Many precursor materialsdecompose when quickly heated to such temperatures.

[0006] Liquid precursors may be vaporized directly using a bubblerdevice. A liquid precursor is heated in a reservoir to a temperature atwhich there is sufficient vaporization to maintain a particulardeposition rate. A stream of carrier gas is directed over the precursoror is bubbled through the liquid precursor in the reservoir. The carriergas transports vaporized precursor molecules to a process chamber fordeposition of a CVD thin film. However, many desirable precursormolecules, when heated to a temperature sufficient to maintain aparticular deposition rate will simply decompose in the bubbler.

[0007] It is also possible to dissolve a liquid or solid precursor in asolvent and vaporize the solution directly. (Many desirable precursorsare solids at room temperature). In the vaporizer (the inlet to whichoften contains a needle or small orifice), the solvent and the precursorare quickly heated to the gas phase. One of the problems associated withthis technique is that the high temperatures necessary to quicklyvaporize the solution cause solvent and precursor molecules todecompose. Decomposition of the solvent and precursor molecules withinthe vaporizer typically produces particulates that clog or otherwiseobstruct the delivery lines between the precursor reservoir and theprocess chamber. Obstruction of the delivery lines cause inconsistentdelivery rates of precursor for deposition on the substrate. Inaddition, the conventional CVD solvents used to dissolve such precursorstypically result in CVD processes where the solvent molecules arecarried along with the precursor. Additionally, such solvent moleculeshave a tendency to decompose, further obstructing the delivery lines ormay be deposited on the substrate. Solvent decomposition products, e.g.,carbonates, formed in the thin film results in poor thin film quality.

[0008] As an alternative, liquid or solid precursors may be mixed withor dissolved in a conventional CVD solvent and the solvent containingthe precursor placed in a bubbler device. The solvent containing thedissolved precursor is then heated in a reservoir. As described abovefor liquid precursors, a stream of carrier gas is directed over orbubbled through the solvent. The carrier gas transports the volatileprecursor molecules from the solvent to a process chamber. The advantageof this technique is that most precursor elements or compounds may bevaporized in a bubbler device at lower temperatures than required forsublimation or direct vaporization of the precursor. Additionally,control of mass delivery of the precursor, using a bubbler device, istypically better than other precursor vaporization methods.Unfortunately, available CVD solvents are typically organic compoundspossessing vapor pressures of greater than about 1 Torr at about roomtemperature. Accordingly, volatilized solvent molecules are oftentransported to the process chamber along with the precursor molecules.This problem is exacerbated when temperatures above room temperature areneeded to volatilize sufficient precursor molecules and/or to maintain agiven depositon rate. As a result, solvent molecules or solventdecomposition products are deposited in the thin-film.

[0009] Further, known CVD solvents do not dissolve the range of solidprecursors necessary to form the CVD thin films currently in demand.Moreover, many of the known CVD solvents for precursor materials arecorrosive to the CVD apparatus, the substrate, and/or thin films alreadyformed on the substrate.

[0010] Accordingly, methods and apparatus that take advantage of thebenefits of using a bubbler (i.e., lower temperatures and increasedcontrol of precursor delivery rates), but overcome the limitationsimposed by conventional CVD solvents are needed. CVD methods andapparatus that do not lead to transport of solvent molecules along withthe vaporized precursors are needed. That is, CVD methods and apparatusare needed that include solvents having extremely low or substantiallyno measurable vapor pressure. Additionally, CVD methods and apparatusthat may be used along with a conventional bubbler device technologywould be preferred. In order to increase the range of precursors thatmay be used to deposit CVD thin films, CVD methods and apparatusincluding solvents that exhibit a wide liquid-temperature range and thatare resistant to decomposition at relatively high-temperature levels,are needed. Further, CVD methods and apparatus that include solventsthat are relatively inert and that dissolve a variety of precursormaterials having a wide range of polarities, are needed.

SUMMARY OF THE INVENTION

[0011] In light of the deficiencies of the prior art, the presentinvention provides methods and apparatus for vaporizing and transportingprecursor molecules to a process chamber for deposition of thin films ona substrate. The methods and apparatus may be used with conventional CVDbubbler apparatus. The methods and apparatus include CVD solvents thatcomprise ionic liquids (i.e., liquids comprising ions) that have lowmelting points (i.e., less than about 250° C.), wide liquid temperatureranges (i.e., liquid temperature ranges preferably of at least about100° C. and more preferably of at least about 200° C.), andsubstantially no measurable vapor pressure (i.e., the ionic liquidsolvents are non-volatile). The ionic liquid CVD solvents have a vaporpressure of preferably less than about 1 Torr at about room temperatureand more preferably less than about 0.1 Torr at room temperature.

[0012] For example, the vapor deposition methods and apparatus of thepresent invention include CVD solvents comprising ionic liquids thatsatisfy Formula (1) as follows:

[0013] wherein R₁ is an alkyl and Y⁻ is selected from the groupconsisting essentially of halides, sulfates, nitrates, acetates,nitrites, chlorocuprates, tetrafluoroborates, tetrachloroborates,hexafluorophosphates, [SbF₆]⁻, tetrachloroaluminates, heteropolyanions(e.g., [MO₁₂O₄₀]³⁻), trifluoromethanesulfonates, and mixtures thereof.Preferably, R₁ is an alkyl having a carbon chain of from about 1 carbonatom to about 30 carbon atoms. Alternatively, R₁ may be selected from agroup consisting essentially of methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof.

[0014] The vapor deposition methods and apparatus of the presentinvention may also include CVD solvents comprising ionic liquids thatsatisfy Formula (2) as follows:

[0015] wherein R₁ and R₂ are independently selected from a groupconsisting essentially of alkyls, methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof, and Y⁻ isselected from the group consisting essentially of halides, sulfates,nitrates, acetates, nitrites, chlorocuprates, tetrafluoroborates,tetrachloroborates, hexafluorophosphates, [SbF6]-,tetrachloroaluminates, heteropolyanions (e.g., [Mo₁₂O₄₀]³⁻),trifluoromethanesulfonates, and mixtures thereof. When R₁ or R₂ comprisean alkyl, the alkyl preferably includes a carbon chain comprising fromabout 1 carbon atom to about 30 carbon atoms.

[0016] The vapor deposition methods and apparatus of the presentinvention may also include CVD solvents comprising ionic liquids thatsatisfy Formula (3) as follows:

[0017] wherein R₁, R₂, R₃, and R₄ are independently selected from agroup consisting essentially of alkyls, methyl groups, ethyl groups,propyl groups, isopropyl groups, n-butyl groups, sec-butyl groups,tert-butyl groups, isobutyl groups, pentyl groups, and mixtures thereof,and Y⁻ is selected from a group consisting essentially of halides,sulfates, nitrates, acetates, nitrites, chlorocuprates,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,tetrachloroaluminates, heteropolyanions (e.g., [Mo₁₂O₄₀]³⁻),trifluoromethanesulfonates, and mixtures thereof. When R₁, R₂, R₃, or R₄comprises an alkyl, preferably, the alkyl comprises a carbon chainhaving from about 1 carbon atom to about 30 carbon atoms.

[0018] The vapor deposition methods and apparatus of the presentinvention also include CVD solvents comprising ionic liquids thatsatisfy Formula (4) as follows:

[0019] wherein R₁, R₂, and R₃ are independently selected from a groupconsisting essentially of alkyls, methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof, and Y⁻ isselected from a group consisting essentially of halides, sulfates,nitrates, acetates, nitrites, chlorocuprates, tetrafluoroborates,tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,tetrachloroaluminates, heteropolyanions (e.g., [Mo₁₂O₄₀]³⁻),trifluoromethanesulfonates, and mixtures thereof. When R₁, R₂, or R₃comprises an alkyl, preferably, the alkyl comprises a carbon chainhaving from about 1 carbon atom to about 30 carbon atoms.

[0020] The vapor deposition methods and apparatus of the presentinvention may also include CVD solvents comprising ionic liquids thatsatisfy Formula (5) as follows:

[0021] wherein n is from about 1 to about 10, and Y⁻ is selected from agroup consisting essentially of halides, sulfates, nitrates, acetates,nitrites, chlorocuprates, tetrafluoroborates, tetrachloroborates,hexafluorophosphates, [SbF6]-, tetrachloroaluminates, heteropolyanions(e.g., [Mo₁₂O₄₀]³⁻), trifluoromethanesulfonates, and mixtures thereof.

[0022] The vapor deposition methods of the present invention may includedissolving precursors in solvents comprising ionic fluids that satisfyFormula (6) as follows:

[0023] wherein R₁, R₂, R₃, and R₄ are independently selected from agroup consisting essentially of alkyls, methyl groups, ethyl groups,propyl groups, isopropyl groups, n-butyl groups, sec-butyl groups,tert-butyl groups, isobutyl groups, pentyl groups, and mixtures thereof,and Y⁻ is selected from a group consisting essentially of halides,sulfates, nitrates, acetates, nitrites, chlorocuprates,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,tetrachloroaluminates, heteropolyanions (e.g., [Mo₁₂O₄₀]³⁻),trifluoromethanesulfonates, and mixtures thereof. When R₁, R₂, R₃, or R₄comprises an alkyl, preferably, the alkyl comprises a carbon chainhaving from about 1 carbon atom to about 30 carbon atoms.

[0024] The present invention further includes heating the solventcontaining the dissolved precursor to a temperature at or near thevolatilization temperature of the dissolved precursor. A stream of gasis then directed over or bubbled through the solvent. The gas transportsprecursor molecules from the solvent to a process or deposition chamber(without transporting solvent molecules) to form a thin film on asubstrate, such as a semiconductor wafer.

[0025] The vapor deposition methods of the present invention provide forthe vaporization and transport of a controlled mass of precursormolecules in the vapor phase. Due to the unique CVD solvents used inpracticing the vapor deposition methods of the present invention,solvent molecules are not transported to the process chamber along withthe vaporized precursors. Further, because the vapor deposition methodsof the present invention include solvents that have an extremely low orsubstantially no measurable vapor pressure, the range of precursormaterials that may be vaporized in the solvent without unwanteddecomposition of the solvent or vaporization of the solvent itself isincreased. Additionally, the vapor deposition methods of the presentinvention include use of solvents that may be used with conventionalbubbler device technology and that are non-corrosive. Moreover, becausethe present invention vapor deposition methods use of solvents thatexhibit a wide liquid temperature range (i.e., greater than about 100°C.), there is a significant increase in the range of materials that maybe deposited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a schematic of a conventional chemical vapor depositionsystem.

[0027]FIG. 2 is a schematic of another chemical vapor deposition system.

DETAILED DESCRIPTION

[0028] The vapor deposition methods and apparatus of the presentinvention include chemical vapor deposition (CVD) solvents that compriseionic liquids. Such solvents, in contrast to conventional CVD solvents,possess wide liquid temperature ranges (typically greater than about100° C.) and exhibit substantially no measurable vapor pressure (i.e.,less than about 1 Torr at about room temperature). Further, the presentmethods and apparatus include ionic liquid CVD solvents that dissolve awide variety of precursor materials.

[0029] The methods and apparatus of the present invention furtherinclude ionic liquid CVD solvents that are relatively inert and stable.For example, chloroaluminate ionic liquids are air and water sensitive(i.e., such ionic liquids tend to be unstable in the presence of air orwater), but hexafluorophoshate, and tetrafluoroborate ionic liquids arenot.

[0030] The methods and apparatus of the present invention include ionicliquids that are liquids at ambient temperature so that dissolution ofthe precursor molecules may be accomplished without heating the mixture.As mentioned below, the cation of the ionic liquid CVD solvent may beselected for its effect on the melting point of the ionic liquid as wellas its solvating properties.

[0031] Physical characteristics of the ionic liquid CVD solvents of themethods and apparatus of the present invention may be altered in orderto allow dissolution and vaporization of a wide variety of precursors.As known to those of ordinary skill in the art, adjustment may be madeto the physical properties of a compound to change one or moreparticular characteristics of the compound. For example, substitutingthe cation of an ionic liquid and/or substituting the anion will alterthe ionic liquid's physical properties. As disclosed in MichaelFreemantle, “Designer Solvents,” Chemical and Engineering News, pp.32-37 (March 1 998), the cation portion of an ionic liquid compound islargely responsible for the low melting point of the ionic liquids. Theanion portion of the ionic liquid compound determines (to a largeextent) its chemical properties, such as reactivity and catalyticactivity. Some ionic liquid anions, for example chloroaluminate, exhibitLewis acidity. Ionic liquids comprising such ions may react undesirablywith certain precursor molecules. Other ionic liquid anions such asnitrate are more generally inert and are useful for a wider range ofprecursors. The respective miscibilities of organic compounds, such asprecursor molecules, in ionic liquids can be varied extensively byaltering the chain lengths of alkyl substituents on the ionic liquidcompound cations. Thus it is possible to tailor the solvent propertiesof ionic liquids through the appropriate choice of alkyl substituents.For example, a more non-polar precursor is more soluble in an ionicliquid having more non-polar character (i.e., ionic liquids possessinglarger R groups). Conversely, for example, a polar precursor moleculewill be most soluble in ionic liquids that possess small alkylsubstituents and are more polar. Thus, the methods and apparatus of thepresent invention include ionic liquid CVD solvents that are able todissolve relatively large quantities of a wide variety of precursors.

[0032] Although the vapor deposition methods and apparatus of thepresent invention are primarily discussed with reference to chemicalvapor deposition, it should be understood that the vapor depositionmethods and apparatus may be applicable to any thin film depositiontechnique requiring a source of volatile molecules or precursors. Suchtechniques may include for example, physical vapor deposition, chemicalvapor deposition, metal organic chemical vapor deposition, atmosphericpressure vapor deposition, low pressure chemical vapor deposition,plasma enhanced low pressure vapor deposition, molecular beam epitaxy,and atomic layer epitaxy.

[0033] Likewise, although the vapor deposition methods of the presentinvention are discussed primarily with reference to semiconductorsubstrates or semiconductor wafers, it should be understood that thesubstrate may comprise silicon, gallium arsenide, glass, an insulatingmaterial such as sapphire, or any other substrate material upon whichthin films may be deposited.

[0034] A typical chemical vapor deposition system that can be used toperform the deposition methods of the present invention is shown inFIG. 1. The CVD system includes an enclosed process (deposition) chamber10. As is conventional CVD, the CVD process may be carried out atpressures of from about atmospheric pressure down to about 10⁻³ Torr,and preferably from about 1.0 to about 0.1 Torr. Accordingly, a vacuummay be created in chamber 10 using a pump 12 (e.g., a turbo pump) andbacking pump 14.

[0035] One or more substrates 16 are positioned in the process chamber10. A constant nominal temperature is established for the substrate 16,preferably at a temperature of about 0° C. to about 800° C., and morepreferably at a temperature of about 100° C. to about 500° C. Substrate16 may be heated, for example, by an electrical resistance heater 18 onwhich substrate 16 is mounted. Other known methods of heating thesubstrate 16 may be utilized.

[0036] A precursor is dissolved in an ionic liquid solvent 40 (asdiscussed in detail below). The ionic liquid solvent 40 (having adesired precursor dissolved therein) is stored in a vessel 42. The ionicliquid solvent 40 including the dissolved precursor is heated to or nearthe vaporization point of the dissolved precursor. The bubbler system asshown in FIG. 1 transports vaporized precursor molecules from the ionicliquid solvent to the CVD process chamber 10. Specifically, a carriergas 44 is pumped into vessel 42 and bubbled through the ionic liquidsolvent 40. The carrier gas 44 may comprise any suitable inert gas. Thecarrier gas 44 typically is selected from a group consisting essentiallyof nitrogen, helium, argon, and mixtures thereof.

[0037] The carrier gas 44 distills the dissolved precursor moleculesfrom the ionic liquid solvent 40 and transports the precursor moleculesto the process chamber 10 through line 45 and gas distributor 46.Additional inert carrier gas may be supplied from source 48 as needed toprovide the desired concentration of precursor and to regulate theuniformity of the deposition across the surface of substrate 16. Asshown, a series of valves 50-54 are opened and closed as required todeliver vaporized precursor and carrier gas to the process chamber 10.

[0038] Generally, the precursor is pumped into the process chamber 10 ata flow rate of about 1 sccm (standard cubic centimeters per minute) toabout 1000 sccm. The substrate 16 is exposed to the precursor at apressure of about 0.001 Torr to about 100 Torr for a time period ofabout 0.01 minutes to about 100 minutes depending upon the desiredthickness of the layer being deposited. In the process chamber 10, theprecursor forms a layer on the surface of the substrate 16. Thedeposition rate is temperature dependent and thus, increasing thetemperature of the substrate 16 increases the rate of deposition.Typical deposition rates are about 100 Å per minute to about 10,000 Åper minute. Closing valve 53 discontinues delivery of the carrier gascontaining the precursor to the process chamber 10.

[0039] Alternatively, if more than one precursor is to be dissolved in asolvent and transported to a process chamber, the vapor depositionsystem shown in FIG. 2 may be used. As shown in FIG. 2, the CVD systemmay include an enclosed chemical vapor deposition chamber 110. The CVDprocess may be carried out at pressures of from atmospheric pressuredown to about 10⁻³ Torr, and preferably from about 10 Torr to about 0.1Torr. A vacuum may be created in chamber 110 using turbo pump 112 andbacking pump 114, or simply a backing pump.

[0040] One or more substrates 116 are positioned in process chamber 110.A constant nominal temperature is established for the substrate,preferably at a temperature of about 50° C. to about 500° C. for certainprecursors. Substrate 116 may be heated, for example, by an electricalresistance heater 118 on which substrate 116 is mounted. Other knownmethods of heating the substrate may also be utilized.

[0041] In this process, a first precursor is dissolved in a solvent ofthe present invention to form a first solution 140 and is stored invessel 142. A source of a suitable inert gas 144 is pumped into vessel142 and bubbled through the first solution 140 picking up the firstprecursor and transporting it into chamber 110 through line 145 and gasdistributor 146. Additional inert carrier gas or reaction gas may besupplied from source 148 as needed to provide the desired concentrationof precursor and regulate the uniformity of the deposition across thesurface of substrate 11 6. As shown, a series of valves 150-154 may beopened and closed as required.

[0042] A second precursor may be dissolved in a solvent to form a secondsolution 240. Second solution 240 is stored in vessel 242. A source 244of a suitable inert gas is pumped into vessel 242 and bubbled throughthe second solution 240 picking up the second precursor and carrying itinto chamber 210 through line 245 and gas distributor 246. Additionalinert carrier gas or reaction gas may be supplied from source 248 asneeded to provide the desired concentration of precursor composition andregulate the uniformity of the deposition across the surface ofsubstrate 116. As shown, a series of valves 250-254 are opened andclosed as required.

[0043] Generally, the first and second vaporized precursor molecules arepumped into the process chamber 110 at a flow rate of about 1 sccm toabout 1000 sccm. The respective flow rates of the first and secondprecursors may be varied to provide the desired ratio of first precursorto second precursor in the co-deposited thin film. The substrate 116 istypically exposed to the precursor compositions at a pressure of about0.001 Torr to about 100 Torr for a time of about 0.01 minutes to about100 minutes. In process chamber 110, the first and second precursorswill form an absorbed layer on the surface of the substrate 116. As theco-deposition rate is temperature dependent, increasing the temperatureof the substrate will increase the rate of co-deposition. Typicalco-deposition rates are about 10 Å/min. to about 1000 Å/min. Closingvalves 153 and 253 terminates the carrier gases transporting the firstand second precursors, respectively.

[0044] Various combinations of carrier gases and/or vaporized precursorsmay be used to practice the vapor deposition methods of the presentinvention. The carrier gas and precursors may be introduced into aprocess chamber in a variety of manners, as known to those personsskilled in the art.

[0045] The vapor deposition methods and apparatus of the presentinvention include CVD solvents that comprise ionic liquids. As usedherein, an ionic liquid means a salt compound having the followingcharacteristics: (1) a melting point of less than about 250° C., (2)substantially no measurable vapor pressure (i.e., less than about 1 Torrand preferably less than 0.1 Torr), (3) a liquid range of about at least100° C., and, preferably about at least 200° C., and (4) functions as asolvent for a wide range of desirable CVD precursor elements andcompounds.

[0046] For example, the methods and apparatus of the present inventioninclude CVD solvents comprising ionic liquids that satisfy Formula (1)as follows:

[0047] wherein R₁ is an alkyl and Y⁻ is selected from a group consistingessentially of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,tetrachloroaluminates, heteropolyanions (e.g., [Mo₁₂O₄₀]³⁻),trifluoromethanesulfonates, and mixtures thereof. Preferably, R₁ is analkyl having a carbon chain of from about 1 carbon atom to about 30carbon atoms. Alternatively, R₁ is selected from a group consistingessentially of a methyl group, ethyl group, propyl group, isopropylgroup, n-butyl group, sec-butyl group, tert-butyl group, isobutyl group,pentyl group, and mixtures thereof.

[0048] The methods and apparatus of the present invention also includeCVD solvents comprising ionic liquids that satisfy Formula (2) asfollows:

[0049] wherein R₁ and R₂ are independently selected from a groupconsisting essentially of alkyls, methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof, and Y⁻ isselected from a group consisting essentially of halides, sulfates,nitrates, acetates, nitrites, tetrafluoroborates, tetrachloroborates,hexafluorophosphates, [SbF₆]⁻, tetrachloroaluminates, heteropolyanions(e.g., [Mo₁₂O₄₀]³⁻), trifluoromethanesulfonates, and mixtures thereof.When R₁ or R₂ comprise an alkyl, the alkyl preferably includes a carbonchain comprising from about 1 carbon atom to about 30 carbon atoms.

[0050] The methods and apparatus of the present invention also includeCVD solvents comprising ionic liquids that satisfy Formula (3) asfollows:

[0051] wherein R₁, R₂, R₃, R₄ are independently selected from a groupconsisting essentially of alkyls, methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof, and Y⁻ isselected from a group consisting essentially of halides, sulfates,nitrates, acetates, nitrites, tetrafluoroborates, tetrachloroborates,hexafluorophosphates, [SbF₆]⁻, tetrachloroaluminates, heteropolyanions(e.g., [Mo₁₂O₄₀]³⁻), trifluoromethanesulfonates, and mixtures thereof.When R₁, R₂, R₃, or R₄ is an alkyl, preferably, the alkyl comprises acarbon chain of from about 1 carbon atom to about 30 carbon atoms.

[0052] The methods and apparatus of the present invention also includeCVD solvents comprising ionic liquids that satisfy formula (4) asfollows:

[0053] wherein R₁, R₂, and R₃ are independently selected from a groupconsisting essentially of alkyls, methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof, and Y⁻ isselected from a group consisting essentially of halides, sulfates,nitrates, acetates, nitrites, tetrafluoroborates, tetrachloroborates,hexafluorophosphates, [SbF₆]⁻, tetrachloroaluminates, heteropolyanions(e.g., [Mo₁₂O₄₀]³⁻), trifluoromethanesulfonates, and mixtures thereof.When R₁, R₂, or R₃ comprises an alkyl, preferably, the alkyl comprises acarbon chain of from about 1 carbon atom to about 30 carbon atoms.

[0054] The methods and apparatus of the present invention also includeCVD solvents comprising ionic liquids that satisfy Formula (5) asfollows:

[0055] wherein n is from about 1 to about 10, and Y⁻ is selected from agroup consisting essentially of halides, sulfates, nitrates, acetates,nitrites, tetrafluoroborates, tetrachloroborates, hexafluorophosphates,[SbF₆]⁻, tetrachloroaluminates, heteropolyanions (e.g., [Mo₁₂O₄₀]³⁻),trifluoromethanesulfonates, and mixtures thereof.

[0056] The methods and apparatus of the present invention also includeCVD solvents comprising ionic liquids that satisfy Formula (6) asfollows:

[0057] wherein R₁, R₂, R₃, R₄ are independently selected from a groupconsisting essentially of alkyls, methyl groups, ethyl groups, propylgroups, isopropyl groups, n-butyl groups, sec-butyl groups, tert-butylgroups, isobutyl groups, pentyl groups, and mixtures thereof, and Y⁻ isselected from a group consisting essentially of halides, sulfates,nitrates, acetates, nitrites, tetrafluoroborates, tetrachloroborates,hexafluorophosphates, [SbF₆]⁻, tetrachloroaluminates, heteropolyanions(e.g., [Mo₁₂O₄₀]³⁻), trifluoromethanesulfonates, and mixtures thereof.When R₁, R₂, R₃, or R₄ is an alkyl, preferably, the alkyl comprises acarbon chain of from about 1 carbon atom to about 30 carbon atoms.

[0058] According to the vapor deposition methods of the presentinvention, a desired precursor is dissolved in a volume of a CVD solventcomprising an ionic liquid. The resulting precursor/solvent solution isplaced within a CVD system vessel such as shown in FIGS. 1 or 2. Asreadily determinable by those persons skilled in the art, the particularprecursor are chosen based on the thin-film layer to be deposited. Theionic liquid is chosen based upon its ability to dissolve the desiredprecursor(s) (an ionic liquid capable of dissolving a relatively largequantity of precursor(s) is generally preferred). The vessel containingthe precursor/solvent solution is preferably heated to a temperature ator near the volatilization temperature of the dissolved precursor(s). Astream of carrier gas is directed over or is bubbled through thesolution to distill and transport vapor-phase precursor molecules to aprocess chamber for deposition of a thin film on a substrate.

[0059] The following example is offered to further illustrate a specificvapor deposition method of the present invention. It should beunderstood, however, that many variations and modifications could bemade while remaining within the scope and spirit of the presentinvention.

EXAMPLE

[0060] This is an example of a chemical vapor deposition method of thepresent invention for the formation of a (Ba,Sr)TiO₃thin-layer on asubstrate, using an ionic liquid CVD solvent. Three precursors,Bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium,Bis(2,2,6,6-tetramethyl-3,5-heptanedionato)strontium andBis(2,2,6,6-tetramethyl-3,5-heptanedionato)barium (all available from,e.g., Strem Chemicals, Inc., of Newburyport, Mass.) are individuallydissolved in separate vessels, each vessel containing a quantity of anionic liquid comprising 1-Ethyl-3-methyl-1H-imidazoliumtetrafluoroborate (available from, e.g., Aldrich Chemical Co., Inc., ofMilwaukee, Wis.). As much of each precursor as possible is dissolved inthe solvent. The solvent solutions having the precursors dissolvedtherein, are then placed in separate bubbler vessels (e.g., 142 and 242of FIG. 2; although not shown in FIG. 2, a third analogous vessel andassociated delivery means would be required in this particular example).

[0061] The vessel containing the titanium precursor is heated byconventional means to about 100° C. The vessels containing the strontiumand the barium precursors are heated by conventional means to about 140°C. and 150° C., respectively. A source of carrier gas, e.g., helium, issupplied to each of the three vessels. At a pressure of about 2 Torr,the precursors are carried (in the vapor phase) to a process chamber(e.g., process chamber 110, FIG. 2). The Ba:Sr:Ti ratio of theprecursors delivered to the process chamber is adjusted by changing thevessels' respective carrier gas flow rates, and/or the temperatures ofthe individual vessels.

[0062] Oxygen gas, as an additional reactant, is delivered to theprocess chamber by a separate means (e.g., from source 248 of FIG. 2).The four gases are then combined in a common line (e.g., 145 in FIG. 2)and are released into the process chamber by a gas distributor (e.g.,146 in FIG. 2). TheBis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium,Bis(2,2,6,6-tetramethyl-3, 5-heptanedionato)strontium,Bis(2,2,6,6-tetramethyl-3,5-heptanedionato)barium, and oxygen react toform a (Ba,Sr)TiO₃ thin-film on a heated substrate (e.g., 116 in FIG. 2)within the process chamber. The thickness of the deposited film isdependent upon the deposition time and the substrate temperature, withlonger deposition times and higher substrate temperatures leading toincreased deposition rates.

[0063] Whereas the invention has been described with reference to arepresentative method, it will be understood that the invention is notlimited to those embodiments. On the contrary, the invention is intendedto encompass all modifications, alternatives, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

1. A method for vaporizing reactants for vapor deposition of a thin filmon a substrate, comprising: providing an ionic liquid; dissolving aprecursor in the ionic liquid; and passing a stream of gas through theionic liquid.
 2. The method of claim 1 , further comprising heating theionic liquid to a temperature equal to about a volatilization point ofthe precursor.
 3. The method of claim 1 , further comprisingtransporting vaporized precursor molecules from the ionic liquid to aprocess chamber.
 4. The method of claim 1 , wherein the ionic liquid isof the formula:

wherein R₁ is alkyl and Y⁻ is selected from a group consistingessentially of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,chloroaluminates, bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof.
 5. The method of claim4 , wherein R₁ is an alkyl having a carbon chain comprising from about 1carbon atom to about 30 carbon atoms.
 6. The method of claim 4 , whereinRi is selected from a group consisting essentially of methyl groups,ethyl groups, propyl groups, isopropyl groups, n-butyl groups, sec-butylgroups, tert-butyl groups, isobutyl groups, and pentyl groups.
 7. Themethod of claim 1 , wherein the ionic liquid is of the formula:

wherein R₁ and R₂ are alkyls and Y⁻ is selected from a group consistingessentially of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,chloroaluminates, bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof.
 8. The method of claim7 , wherein R₁ is an alkyl having a carbon chain comprising from about 1carbon atom to about 30 carbon atoms.
 9. The method of claim 7 , whereinR₁ and R₂ are independently selected from a group consisting essentiallyof alkyls, methyl groups, ethyl groups, propyl groups, isopropyl groups,n-butyl groups, sec-butyl groups, tert-butyl groups, isobutyl groups,and pentyl groups.
 10. The method of claim 1 , wherein the ionic liquidsatisfies the formula:

wherein R₁, R₂, R₃, R₄ are alkyls and Y⁻ is selected from a groupconsisting essentially of halides, sulfates, nitrates, acetates,nitrites, tetrafluoroborates, tetrachloroborates, hexafluorophosphates,[SbF₆]⁻, chloroaluminates, bromoaluminates, chlorocuprates,heteropolyanions, trifluoromethanesulfonates, and mixtures thereof. 11.The method of claim 10 , wherein R₁ is an alkyl having a carbon chaincomprising from about 1 carbon atom to about 30 carbon atoms.
 12. Themethod of claim 10 , wherein R₁, R₂, R₃, and R₄ are independentlyselected from a group consisting essentially of alkyls, methyl groups,ethyl groups, propyl groups, isopropyl groups, n-butyl groups, sec-butylgroups, tert-butyl groups, isobutyl groups, pentyl groups, and mixturesthereof.
 13. The method of claim 1 , wherein the ionic liquid satisfiesthe formula:

wherein R₁, R₂, and R₃are alkyls and Y⁻ is selected from a groupconsisting essentially of halides, sulfates, nitrates, acetates,nitrites, tetrafluoroborates, tetrachloroborates, hexafluorophosphates,[SbF₆]⁻, chloroaluminates, bromoaluminates, chlorocuprates,heteropolyanions, trifluoromethanesulfonates, and mixtures thereof. 14.The method of claim 13 , wherein R₁, R₂, and R₃ are independentlyselected from a group consisting essentially of alkyls having carbonchains comprising from about 1 carbon atom to about 30 carbon atoms. 15.The method of claim 14 , wherein R₁, R₂, and R₃ are independentlyselected from a group consisting of alkyls, methyl groups, ethyl groups,propyl groups, isopropyl groups, n-butyl groups, sec-butyl groups,tert-butyl groups, isobutyl groups, and pentyl groups.
 16. The method ofclaim 1 , wherein the ionic liquid satisfies the formula:

wherein n is from about 1 to about 10 and Y⁻ is selected from a groupconsisting essentially of halides, sulfates, nitrates, acetates,nitrites, tetrafluoroborates, tetrachloroborates, hexafluorophosphates,[SbF₆]⁻, chloroaluminates, bromoaluminates, chlorocuprates,heteropolyanions, trifluoromethanesulfonates, and mixtures thereof. 17.The method of claim 1 , wherein the ionic liquid satisfies the formula:

wherein R₁, R₂, R₃, R₄ are alkyls and Y⁻ is selected from a groupconsisting essentially of halides, sulfates, nitrates, acetates,nitrites, tetrafluoroborates, tetrachloroborates, hexafluorophosphates,[SbF₆]⁻, chloroaluminates, bromoaluminates, chlorocuprates,heteropolyanions, trifluoromethanesulfonates, and mixtures thereof. 18.The method of claim 17 , wherein R₁ is an alkyl having a carbon chaincomprising from about 1 carbon atom to about 30 carbon atoms.
 19. Themethod of claim 17 , wherein R₁, R₂, R₃, and R₄ are independentlyselected from a group consisting essentially of alkyls, methyl groups,ethyl groups, propyl groups, isopropyl groups, n-butyl groups, sec-butylgroups, tert-butyl groups, isobutyl groups, pentyl groups, and mixturesthereof.
 20. A method for vapor deposition of a thin film on asubstrate, the method comprising: providing an ionic liquid includingone or more precursors; heating the ionic liquid; transporting theprecursor in the vapor phase from the ionic liquid to a substrate; anddepositing the precursor on the substrate.
 21. The method of claim 20 ,wherein the precursor is dissolved in the ionic liquid.
 22. The methodof claim 20 , wherein the vapor-phase precursor is distilled from theionic liquid and transported to the substrate by a carrier gas.
 23. Amethod for vaporizing reactants for vapor deposition of a thin film on asubstrate, comprising: dissolving a precursor in a solvent thatsatisfies the formula:

wherein R₁ is an alkyl and Y⁻ is selected from a group consistingessentially of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,chloroaluminates, bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof; and bubbling a streamof gas through the solution containing the precursor to distillprecursor molecules in the vapor phase from the solution.
 24. A methodfor vaporizing reactants for vapor deposition of a thin film on asubstrate, comprising: dissolving a precursor in a solvent thatsatisfies the formula:

wherein R₁ and R₂ are alkyl and Y⁻ is selected from the group consistingof halides, sulfates, nitrates, acetates, nitrites, tetrafluoroborates,tetrachloroborates, hexafluorophosphates, [SbF₆]⁻, chloroaluminates,bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof; and bubbling a streamof gas through the solution containing the precursor to distillprecursor molecules in the vapor phase from the solution.
 25. A methodfor vaporizing reactants for vapor deposition of a thin film on asubstrate, comprising: dissolving a precursor in a solvent thatsatisfies the formula:

wherein R₁, R₂, R₃, R₄ are alkyl and Y⁻ is selected from the groupconsisting of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,chloroaluminates, bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof; and bubbling a streamof gas through the solution containing the precursor to distillprecursor molecules in the vapor phase from the solution.
 26. A methodfor vaporizing reactants for vapor deposition of a thin film on asubstrate, comprising: dissolving a precursor in a solvent satisfyingthe formula:

wherein R₁, R₂, and R₃ are alkyl and Y⁻ is selected from the groupconsisting of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]-,chloroaluminates, bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof; and bubbling a streamof gas through the solution containing the precursor to distillprecursor molecules in the vapor phase from the solution.
 27. A methodfor vaporizing reactants for vapor deposition of a thin film on asubstrate, comprising: dissolving a precursor in a solvent thatsatisfies the formula:

wherein R₁, R₂, R₃, R₄ are alkyl and Y⁻ is selected from the groupconsisting of halides, sulfates, nitrates, acetates, nitrites,tetrafluoroborates, tetrachloroborates, hexafluorophosphates, [SbF₆]⁻,chloroaluminates, bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof; and bubbling a streamof gas through the solution containing the precursor to distillprecursor molecules in the vapor phase from the solution.
 28. A methodfor vaporizing reactants for vapor deposition of a thin film on asubstrate, comprising: dissolving a precursor in a solvent thatsatisfies the formula:

wherein n is from 1 to 10 and Y⁻ is selected from the group consistingof halides, sulfates, nitrates, acetates, nitrites, tetrafluoroborates,tetrachloroborates, hexafluorophosphates, [SbF₆]⁻, chloroaluminates,bromoaluminates, chlorocuprates, heteropolyanions,trifluoromethanesulfonates, and mixtures thereof; and bubbling a streamof gas through the solution containing the precursor to distillprecursor molecules in the vapor phase from the solution.
 29. Anapparatus for vaporizing and transporting precursor molecules to adeposition chamber for deposition of a thin film on a substrate, theapparatus comprising: a vessel containing an ionic liquid; a carrier gassource in fluid communication with the vessel; and a deposition chamberin fluid communication with the carrier gas source.
 30. An apparatus forvaporizing and transporting precursor molecules to a deposition chamberfor deposition of a thin film on a substrate, the apparatus comprising:a vessel containing an ionic liquid having a precursor dissolvedtherein; a bubbler device for bubbling a carrier gas source through thevessel; and a gas line for transporting carrier gas and vaporizedprecursor molecules from the vessel to the deposition chamber.
 31. Anapparatus for vaporizing and transporting precursor molecules to adeposition chamber for deposition of a thin film on a substrate, theapparatus comprising: an ionic liquid source; a carrier gas source influid communication with the ionic liquid source; and a depositionchamber in fluid communication with the carrier gas source.
 32. Anapparatus for vaporizing and transporting precursor molecules to adeposition chamber for deposition of a thin film on a substrate, theapparatus comprising: an ionic liquid source; a carrier gas source; abubbler device for delivering the carrier gas source to the ionic liquidsource; and a deposition chamber in fluid communication with the ionicliquid source to receive vaporized molecules from the ionic liquidsource.